The increasing demand for electrical power has forced electrical utilities to burn increasing quantities of fossil fuels such as coal and oil. However, electric utilities also face increasing environmental standards imposed upon their operations by state and federal regulatory agencies that mandate reduced particulate and acid generating smoke stack emissions. To reduce acid generating emissions, electrical utilities have turned to burning low-sulfur coal in their boilers to generate the steam necessary for electric power generation. To reduce the particulate emissions, electric utilities generally use a flue gas treatment system to remove a majority of the particulate matter in the gas effluent passing out of the smoke stack. Such flue gas treatment systems typically comprise an electrostatic device such as an electrostatic precipitator or a fabric filter baghouse to remove the particulate. Such devices also typically provide a source of conditioning agent to the flue gas to enhance the effectiveness of the precipitator or filter in removing the particulate.
The efficiency of an electrostatic precipitator in removing particulate matter from the boiler flue gas is partially dependent upon the electrical resistivity of the entrained particulate matter in the boiler flue gas. The entrained particulate matter expelled from a boiler fired with low-sulfur coal, i.e., coal having less than 1 percent sulfur, has been found to have a resistivity of approximately 10.sup.13 ohm/cm. It has been determined that the most efficient removal of particulate matter by electrostatic precipitation occurs when the particulate matter resistivity is approximately 10.sup.8 ohms/cm. Therefore, to obtain more effective use of an electrostatic precipitator, the resistivity of the entrained particulate matter from low-sulfur content coal must be reduced. Electrical utilities have long used conditioning agents introduced into the flue gas flow upstream of the electrostatic precipitator to reduce the resistivity of the entrained particles. Various chemicals, such as water, hydrous ammonia, sulfuric acid, sulfur trioxide, phosphoric acid and various ammonia-bearing solutions have been used as conditioning agents.
Similarly, the operating pressure of a fabric filter baghouse will decrease with the addition of humidity to the flue gas which increases ash cohesivity and dust cake porosity. Thus, the pressure drop across the baghouse is reduced by the introduction of humidity to the flue gas.
Water has been recognized for over thirty years as a primary potential conditioning agent for electrostatic precipitators. For example, U.S. Pat. No. 2,864,456 discloses varying the amount of conditioning agent such as water and varying the voltage of the electrostatic precipitator to maintain the optimum sparking level for particulate removal. In addition, U.S. Pat. No. 3,665,676 discloses that mixtures of water and ammonia or ammonium salts such as ammonium sulfate and ammonium bisulfate are effective as flue gas conditioning agents.
However, while several commercial attempts have been made to utilized the simple concept of water injection to increase the efficiency of flue gas precipitators, most commercial scale attempts have not been satisfactory for technological reasons. To be effective as a conditioning agent, water or water mixture must be atomized to a very fine mist that can be evaporated by the resident heat in the flue gas in a very short distance from its introduction. Water droplets that do not evaporate can mix with the ash particulate in the flue gas forming a "mud" or "sludge" that collects on any mechanical component in its path. After a very short time, the water will be evaporated from the coating, leaving a very hard, cement like coating on the component. Over time, the coating can increase to the point where it obstructs the flow path of the flue gas or reduces the operational performance of the precipitator by reducing the critical electrical clearances. Prior art systems were ineffective because economic and technological solutions did not exist to produce sufficiently atomized water to avoid the "sludge" problem.
In order to atomize the water or water mixture sufficiently to a droplet size that will evaporate rapidly enough to avoid formation of sludge coatings, it has been found that the water droplets must be 50 microns or less. A significant amount of energy is required to be applied to the atomization system to overcome the cohesive attraction of individual water molecules to reduce the droplet size to below 50 microns. Droplet size is inversely proportional to the amount of energy introduced into the system.
While there have been significant advances in nozzle technology that allow the production of water droplets smaller than 50 microns, large atomizing air compressors or steam generating units are required to produce the energy necessary to force the water through such nozzles with sufficient velocity to produce extremely fine droplets. Consequently, while the art has developed to the point where it is technologically feasible to produce water mist with droplets less than 50 microns, the energy required makes such a system economically undesirable as a flue gas conditioning system for electrical utilities.
Thus, it would be a substantial advance in the art to have a system for treating boiler flue gas to improve the removal of particulate matter that utilizes highly atomizer water or water mixtures as a conditioning agent that is effective and economically acceptable. Accordingly, a system for treating boiler flue gas to improve the removal of particulate matter that utilizes highly atomized water that taps available energy sources within the system to increase the energy level of the water to facilitate atomization in an effective and economically feasible manner would overcome the deficiencies in the prior art.